1. Field of the Invention
The present invention generally relates to devices for measuring electric current waveforms and differential probes, and particularly relates to a device for measuring electric current waveforms and a differential probe wherein the differential probe is designed to measure waveforms of high-speed electric currents flowing through wires by using electro-optical effect.
2. Description of the Related Art
In designing and manufacturing of electrical circuits on printed circuit boards, it is vitally important to measure waveforms of electric currents flowing through wires of the circuits. To this end, various schemes are available for measuring electric currents. A typical method is one that detects an induced magnetic field generated by an electric current.
FIG. 18 is a schematic drawing for explaining a method of current detection by use of an electric probe utilizing a Hall device, which is a typical example of a device that detects an induced magnetic field.
An induced magnetic field that is generated by an electric current flowing through a wire 61 is intensified by a magnetic material ring 62. The magnetic material ring 62 has a Hall device 63 integrated therein, which works with a detection amplifier 64 to detect the induced magnetic field based on the principle of the Hall effect.
In this case, however, presence of a gap in the magnetic material ring 62 results in a significant decrease in detection sensitivity, so that the magnetic material ring 62 needs to fully circle around the wire 61. Because of this, the wire 61 that is formed as part of a Cu pattern on a printed circuit board has to be cut, and a lead line has be to be led from the wire 61.
Since the lead line has its own inductance, the lead line undesirably affects circuit operation if the operation speed is high. This makes it impossible to measure waveforms of high-speed electric currents.
Another method for measuring electric currents is to measure a current from a voltage drop by utilizing the fact that a current flowing through a resistor can be measured from a voltage drop and the resistance. Wires in circuits, however, do not have sufficient resistances to allow a detectable voltage drop to develop. In this case, a resistor needs to be inserted, and a voltage drop between the two end points of the resistor is detected.
When such a voltage drop is measured between the two end points of a resistor device, potentials may be detected at both ends, and a difference between the detected potentials may be obtained thereafter. Such measurement does not provide a true voltage drop waveform in a strict sense since the two measurements are not obtained simultaneously.
For the purpose of measuring an electric current waveform, therefore, a differential probe is often used. In particular, a FET differential probe, which has high input impedance, is useful in measuring high-speed signals.
Electrical differential probes such as FET differential probes, however, are susceptible to error that is caused by asymmetry of circuit structures. Such error may be miniscule, but cannot be ignored since a differential signal to be detected has small amplitudes in comparison with amplitudes of common-mode signals.
Further, since an electrical measurement system is connected to a target system to be measured, the input impedance of a probe decreases in a high-frequency range, affecting the target system to a noticeable degree.
To obviate the problem of input impedance, electro-optical crystal having the Pockels effect may be utilized.
Such electro-optical crystal includes crystal of a vertical type and crystal of a horizontal type. The crystal of a vertical type has a high sensitivity to electric fields that are parallel to the optical axis of passing light, and the crystal of a horizontal type has a high sensitivity to electric fields that are perpendicular to the optical axis. As an example of use of the electro-optical effect of vertical-type electro-optical crystal, a voltage level is measured based on the amount of polarization by detecting the polarization of a laser beam when the laser beam passes through or is reflected by electro-optical crystal in a configuration in which the laser beam is directed to the electro-optical crystal situated in proximity of a measurement point. (See J. A. Valdmanis and G. Mourou, IEEE Journal of Quqntum Electronics, Vol. QE-22, 1986, pp. 69-78.)
FIG. 19 is a schematic diagram for explaining a method of measuring a voltage level by use of a vertical-type electro-optical crystal 71 such as ZnTe, Bi12SiO20, or the like.
The vertical-type electro-optical crystal 71 has a surface thereof provided with a reflection electrode 72, on which a probe needle 73 is situated. The other surface of the vertical-type electro-optical crystal 71 has a transparent electrode 74 provided thereon for receiving a reference voltage.
The probe needle 73 comes in contact with a wire 75 formed on a circuit board 76, so that a target signal 77 is applied to the vertical-type electro-optical crystal 71 via the probe needle 73. Changes in the applied target signal 77 are detected as the amount of polarization of a laser beam 78.
This method of measuring a voltage level by use of the vertical-type electro-optical crystal 71, however, is directed to measurement of a voltage level applied to a wire. Namely, this method cannot be directly applied to measurement of high-speed electric currents that pass through wires in a circuit that is formed on a printed circuit board.
Further, in respect of the vertical-type electro-optical crystal 71, a travel direction of the laser beam 78 is the same as a direction of a detectable electric field. If the thickness of the vertical-type electro-optical crystal 71 is decreased in order to intensify the electric field, interaction between the laser beam 78 and the vertical-type electro-optical crystal 71 will become weaker. It is thus difficult to step up detection efficiency.
FIG. 20 is a schematic diagram for explaining a method of measuring a voltage level by use of a horizontal-type electro-optical crystal 81 such as LiNbO3, LiTaO3, or the like. (See Japanese Patent Laid-open Application No. 6-27154.)
In FIG. 20, a target voltage E to be measured is applied between nodes a and b of a bridge circuit, and the horizontal-type electro-optical crystal 81 is connected between nodes c and d of the bridge circuit. Light beams from opposite-phase light sources D1 and D2 are directed to photosensitive resistors R1 and R2 via optical fibers 82 and 83, respectively. A light beam from a light source D3 of a measurement unit 86 is directed to and passes through the horizontal-type electro-optical crystal 81, and the output beam having polarization thereof changed according to the Pockels effect is detected by a photoelectric device PD.
This method of measuring a voltage level is quite peculiar, and merely offers a schematic configuration. In practice, this method cannot be applied to measurement of high-speed electric currents despite the high-impedance feature of the method.
As described above, there is no method to date that measures, with sufficient accuracy by use of a simple configuration, high-speed electric currents as they flow through wires.
Accordingly, there is a need for a scheme that can measure high-speed electric currents with sufficient accuracy by use of a simple configuration.
It is a general object of the present invention to provide a device that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.
It is another and more specific object of the present invention to provide a device that can measure high-speed electric currents with sufficient accuracy by use of a simple configuration.
In order to achieve the above objects according to the present invention, a device for measuring an electric current of a target circuit includes a pair of contact pins, a resistor electrically connected between the contact pins, a voltage drop appearing across the resistor when the contact pins come into contact with the target circuit to direct the electric current to the resistor, and an electro-optical crystal electrically connected between the contact pins in parallel with the resistor, the electro-optical crystal having a voltage applied thereto responsive to the voltage drop appearing across the resistor, wherein the voltage applied to the electro-optical crystal changes polarization of a light beam passing therethrough, thereby allowing the electric current to be measured from the polarization.
In the device as described above, the resistor having a small resistance that would not affect circuit operation is inserted into the target circuit, and the voltage drop is detected by measuring polarization statuses of the light beam. This method can, by its nature, avoid error that would be caused by asymmetry between two nodes if an electric probe such as an FET probe or the like was employed.
Further, since the electro-optical crystal is electrically independent of the detection system, input impedance for the detection system is practically infinite, thereby not affecting the detection system at all.
When a horizontal-type electro-optical crystal is used as the electro-optical crystal, an incident direction of the light beam is perpendicular to the direction of a detectable electric field, so that the thickness of the electro-optical crystal can be freely altered to adjust the degree of interaction under the condition of a constant electric field.
Further, the electro-optical crystal and the resistor are arranged in parallel between the contact pins, which serve as contact nodes that come into contact with the target circuit. The probe having such a configuration makes it easier to measure electric-current waveforms with respect to various types of circuit wires.
Since the electro-optical crystal typically has natural birefringence characteristics, an optical device may be provided for the purpose of compensating for the birefringence characteristics along the travel direction of a laser beam.
The horizontal-type electro-optical crystal made of LT (LiTaO3), LN (LiNbO3), or the like has birefringence characteristics, which make returning light have a different phase from that of incident light even in the absence of applied voltage. Because of this reason, it is desirable to provide a compensation-purpose optical device for compensating for natural birefringence. In general, a horizontal-type electro-optical crystal the same size as the electro-optical crystal is provided at a 90xc2x0 angle around the optical axis, thereby canceling the birefringence.
Moreover, it is preferable to provide a heater for the compensation-purpose optical device.
In general, a temperature factor of the electro-optical crystal is significant, so that a phase difference indicative of polarization statuses varies depending not only on the applied voltage but also on temperature. Therefore, an effect of temperature variation on the electro-optical crystal needs to be cancelled when the resistor for generating the applied voltage generates heat. To this end, a heater is provided.for the compensation-purpose optical device so as to resolve the temperature dependency.
Further, a heat-control unit is preferably provided for the purpose of controlling the heater such that signals detected under the conditions of a particular phase have the same magnitude.
Since an average of the target electric signal is unknown, the control of the heater is preferably conducted such that the signals detected under the condition of the same reference phase have the same magnitude.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.